This introduction presents an overview of the key concepts discussed in this book. The book draws a distinction between "world cities" and "megacities" these – two concepts derived from urban geography. It critically examines and readdresses sociolinguistic theory. The book seeks to achieve this by adapting sociolinguistic theory to the language life of individuals in large urban centers. Language in the city provides for ample opportunity to challenge and expand sociolinguistic theory, which has predominantly drawn from western cases and ethnic groups. Sociolinguistic research, of which William Labov is generally considered one of the influential instigators, has yielded highly useful explanations for the language use of groups of people who share certain characteristics. In studying situated language life in 12 urban ecologies, the book seeks to shed light on world cities and megacities as sociolinguistic systems in their own right, the range and nature of transformations going on there, differences and congruencies between the world cities and megacities.